Taxine. Part VI. The stereochemistry of taxicin-I and taxicin-II

By transforming one of them into a compound of known stereochemistry, configurations are deduced for two cleavage fragments corresponding to ring C of the taxicin-I molecule. Chemical, optical, and n.m.r. observations then show that in the intact taxicin molecule ring C is essentially chair-shaped, and that the eight-membered ring B is in a “boat-chair” conformation. This requires a trans-B/C ring-junction, and also permits the configurations at positions 2, 9, and 10 to be deduced from the n.m.r. data, leading to the complete stereostructures for 4,16-dihydrotaxicin-I and 4,16-dihydrotaxicin-II. Some n.m.r. observations on taxicin derivatives are discussed in the light of these results. It is suggested inter alia that the effect which causes anomalous differences in the shielding of the α-proton in axial and equatorial α-halogenocyclohexanones may be paralleled in chair-shaped α-acyloxy-cyclohexanones, of which the oxonortaxicins provide examples.